Oscillating-piston drive for a vacuum pump and an operating method for said drive

ABSTRACT

An oscillating-piston drive for a vacuum pump ( 1 ) with a piston ( 2 ), which presents has two piston sections ( 3,4 ) and an intermediate zone provided with a drive magnet ( 11 ). Cylinder sections ( 8, 9 ) slidingly receive the piston sections ( 3, 4 ). An annular recess ( 12 ) is defined between the cylinder sections ( 8, 9 ) at a central yoke ( 19 ). The recess provides space for movement of the drive magnet ( 11 ). An electromagnetic drive which surrounds the piston ( 2 ) includes yoke components ( 17, 18, 19 ) and coils ( 15, 16 ) situated to the sides of said central yoke. Negative influences on the delivery rate of the pump are reduced by a can ( 34 ) which delimits the recess ( 12 ) peripherally or by controlling the current supply to the coils, such that only one coil conducts current at a time.

The invention relates to an oscillating-piston drive for a vacuum pumpwith a piston, which presents two piston sections and an intermediatezone provided with a drive magnet, cylinder sections associated to saidpiston sections, an annular recess arranged between the cylindersections at a central yoke, said recess forming the space for movementof said drive magnet and an electromagnetic drive surrounding thepiston, which comprises yoke components and coils situated to the sidesof said central yoke. Moreover, the present invention relates to anoperating method for the drive.

Generally it is the aim of the here affected developers and designers toimprove the delivery rate or effect (pumping capacity, compression) of avacuum pump while simultaneously maintaining or even reducing, ifpossible, the construction volume and/or preferably even reducing energyconsumption. This aim is equivalent in that in the course of the furtherdevelopment, respectively design of a pump of the affected type,measures which become necessary must not be associated with impairmentsaffecting the delivery rate.

It is the task of the present invention to propose an oscillating pistondrive for a vacuum pump in which delivery rate impairments are reduced.

This task is solved through the present invention through thecharacterising features of the patent claims.

An oscillating piston drive of the aforementioned kind is known from WO00/63 556, drawing FIG. 8. It exhibits a number of components (coils,pole components, cylinders etc.), adjacent with respect to the space formovement of the drive magnet and which have an influence on the deliveryrate. There exists the risk that the space for movement be linked bymeans of slots between the components or current feedthroughs to theouter surroundings. Through such slots, air enters into the space formovement and increases the low pressure forming during operation of thepump within the space for movement. Measures for sealing these slots(for example, adhesive or sealant layers) may impair the efficiency ofthe electromagnetic drive, since these will increase the distancebetween the individual components.

By means of a first solution for the posed task, it is proposed that acan delimits peripherally the outer recess. A pipe section peripherallydelimiting the space for movement of the drive magnet reduces the numberof slots opening out into the space for movement so that the risk ofunwanted pressure increases in this volume is substantially removed. Thewall thickness of the pipe may be very small, below 1 mm, for example,so that impairments in the efficiency of the electromagnetic drive arenegligible.

Expedient materials for the can are those which offer good slidingproperties, like plastic, aluminium, stainless steel¹⁾ or alike (not—oronly weakly ferromagnetic)²⁾.¹⁾ Translator's note: The German text states “Edelstahlt” here whereas“Edelstahl” would be appropriate. Therefore the latter has been assumedfor the translation.²⁾ Translator's note: In the German text the right bracket is missing.It has been added to the translation.

Alternatively the can may consist of a more strongly ferromagneticmaterial, and its wall thickness selected at least in the area of thesections outside the central yoke such that the drive magnet magnetisesthe respective section to the saturation point when it is located in thezones outside of the central yoke. This embodiment of the can has theeffect that it at least partly becomes part of the drive. The in eachinstance saturated section is practically no longer existent for themagnetic field of the related coil. This has an effect equivalent to anenlargement of the air gap for this coil and results in a reduction inthe inductance of specifically this coil. The current in a coil is builtup when the drive magnet is located in the area of this coil, i.e. thecorresponding can section is saturated. Lower inductance means fastercurrent build-up at a given voltage. With the magnetic field of thiscurrent, now the drive magnet of this coil is repelled towards theaxially opposing coil. The saturation effect of the drive magnet on thecan disappears. But since the current now has the required level, theincrease in inductance is not disturbing.

This operating principle requires that the magnetic field of the drivemagnet be stronger compared to that of the coil. If this were not thecase, then the field of the coil would practically “overwrite” the fieldof the drive magnet in the can (the directions of the fields oppose eachother) thereby cancelling the saturation immediately. In the instance ofthis drive, the necessary forces may, however, only be implemented bysufficiently strong rare earth magnets, for example. In the instance ofthese magnets this requirement is always fulfilled.

From the above descriptions it is apparent that it is expedient tocontrol the current through the coils such that a current is allowed toflow only through one coil at a time. In this manner it is achieved thatthe current in one coil is built up precisely when the drive magnet islocated in the area of this coil.

Controlling the drive by means of semiconductor switches allows theavoidance of further losses. To explain this improvement, the existenceof a linear drive in accordance with drawing FIG. 8 of WO 00/63 556 isagain assumed. In the instance of this linear drive, the magnetic fieldof one of the two coils will only generate a force on the pistonprovided the corresponding piston section is located in the area of therespective coil. The other—current carrying—coil is ineffective duringthis time. When commonly letting a current flow through both coilssimultaneously thus higher losses in the coils are created as would benecessary for producing the forces. Moreover, letting a current flowsimultaneously through both coils implies that the drive electronicsmust be capable of switching on and off both polarities of the current.This not only increases the power loss but also the complexity for thedrive electronics.

In a second solution for the task of the present invention it isproposed that only one current polarity be assigned to each of thecoils, i.e. the positive current polarity is assigned to one coil andthe negative current polarity to the other. For example, the twopolarities of the 50 Hertz mains AC can be “distributed” to both coils.

This may be implemented with a simple thyristor regulator. The currentamplitude of each half-wave may be adjusted by means of a simple,cost-effective phase angle regulator, as is known from electric drillingmachines, for example.

The input signal for the phase angle regulator may for example be

-   -   defined at a fixed value for a pump application (pressure, mains        voltage, number of stages)    -   made to depend on the mains voltage,    -   defined by a sensor for detecting the position of the piston or    -   defined by a sensor for observing the valve movement.

The frequency of the piston's stoke will in all cases result from thefrequency of the supplied alternating current.

The advantages of these measures are on the one hand that losses in thecoils are reduced, since the current is allowed to flow only through onecoil at a time. Also the implementation of the control electronics ismore simple, since it is no longer required for the currents to flowsimultaneously through both coils.

Further advantages and details of the present invention shall beexplained with reference to the examples of embodiments depicted in thedrawing FIGS. 1 to 3.

Depicted is in

-   -   drawing FIG. 1, a sectional view through a piston vacuum pump        equipped with a drive in accordance with the present invention,    -   drawing FIG. 2, a partial sectional view at the level of the can        and    -   drawing FIG. 3, a schematic representation of a pump in        accordance with the present invention with means for supplying        the drive coils with current.

The drawing figures each depict a piston vacuum pump 1 with a piston 2.This exhibits piston sections 3 and 4, to the unoccupied face sides ofwhich each a cylindrical pump chamber 5, respectively 6 is assigned. Thepiston 2 and the pump chambers 5, 6 are located in a housing 7 withcylinder sections 8, 9 for the piston sections 3, 4. The materials forthe sliding cylinder surfaces and the corresponding piston surfaces areselected in a basically known manner such that the pump may be operateddry, i.e. without lubricant.

A linear drive is assigned to the piston 2. Said linear drive compriseson the side of the piston a permanent magnet ring 11, encompassing thepiston 2 at its central zone. The permanent magnet ring 11 moves in theannular volume (recess 12) encompassing the piston 2. On the statorside, further permanent magnet rings 13, 14 are assigned to thepermanent magnet 12 on the side of the piston, said further permanentrings axially delimiting the annular recess 12. At the level of thesepermanent magnetic rings, also cylinder sections 8, 9 terminate.

Moreover, the coils 15 and 16 are components of the linear drive on thestator side. These are partly encompassed by yoke components 17, 18 andjointly with these yoke components said coils encompass the cylindersections 8, respectively 9. Located between the coils 15, 16 and theyoke components 17, 18 is an annular center yoke 19, the inner surfacesof which face the annular chamber 12. Currents are made to flow throughcoils 15, 16 such that the magnetic field produced by the coils andguided by the yoke components 17 to 19 interact with the magnetic fieldsof the permanent magnet rings 11, 13 and 14 in the desired manner. Thepiston 2 shall oscillate about its centre position such that during thismovement the face sides of the piston may fulfil their pumping function.

For the purpose of fulfilling the desired pumping effect, thecompression chambers 5, 6 are each equipped with an inlet valve and anoutlet valve (only depicted in drawing FIG. 1). To each of the inletvalves there is associated an inlet aperture 21, respectively 22 whichis each located between an outer inlet chamber 23, respectively 24 andthe corresponding pump chamber 5, respectively 6. The inlet apertures21, 22 are designed by way of slot-like radially extending openings inthe respective cylinder wall 8, respectively 9. The piston sections 3and 4 release the respective inlet aperture when assuming one of theirtwo dead centres (each in the retracted position in the cylindersection). The outlet valves 26, 27 are located at the respective facesides. There closure components 28, 29 separate the respectivecompression chamber 5, respectively 6 from an outlet chamber 31, 32 solong until they are opened by the respective piston section 3,respectively 4—at high pressure differences also by the generatedpressure. The closure components 28, 29 are designed by way of flexiblediscs extending over the entire cross-section of the cylinder sections3, 4, said disks being centrally affixed at the housing 7 and which areperipherally actuated by the produced pressure or the face sides of thepiston 2. To this end, the piston face sides have been designed to havea concave contour. The face sides of the components forming the cylindersections 8, 9 have the function of the valve seats.

In all, two compression stages are present. They may be operated inseries or in parallel. Details on this are not presented.

In all drawing figures the can is designated as 34. It encompasses theannular chamber, respectively the recess 12, and extends into the areaof the stator permanent magnet rings 13, 14.

Drawing FIG. 2 depicts that the can 34 exhibits two lateral sections 35,36 of relatively small wall thickness and a centre section with agreater wall thickness. The wall thickness of the lateral sections 35,36 is below 1 mm, preferably 0.7 mm. At these wall thicknesses, thedesired saturation through the drive magnet 11 occurs, provided thedrive magnet is located in the vicinity of the sections 35, 36. Thegreater wall thickness in the centre zone is only required when the canneeds to offer a sufficient degree of mechanical strength.

Drawing FIG. 3 depicts the vacuum pump 1 with its linear drive only in ahighly schematic manner. Additionally depicted is an embodiment for thepower supply in accordance with the present invention for the coils 15,16. Through the connection 41 an alternating current, preferably themains current at 50 Hz is supplied to two thyristor regulators 42, 43,of which each is connected to one coil 15, respectively 16. Regulator 42allows the passage only of the positive half-wave, regulator 43 allowsthe passage of only the negative half-wave of the alternating current.Passing of currents through the coils is thus no longer effectedsimultaneously but alternatingly at only one of the two currentpolarities. The current/time diagrams 44, 45, 46 presented in eachinstance the area of the current feed and between the regulators 42, 43and the coils 15, 16, render apparent the power supply in accordancewith the present invention.

Expediently the coils 15, 16 are switched on in the respective supplycircuit in such a manner that they effect repelling forces on the drivemagnet 11. Thus the piston will oscillate about its central position atthe frequency of the supplied alternating current.

The permanent magnets 13, 14 are expediently magnetised such that theywill effect on the drive magnet a repelling action. This solution offersthe advantage that mechanical springs which move the piston back to itscentral position can be omitted.

1. An oscillating piston drive for a vacuum pump comprising: a pistonhaving two piston sections and a central zone; a drive magnet mounted inthe central zone; cylinder sections in which the piston sections areslidably received; an annular recess located between the cylindersections at the level of a central yoke said recess receiving andforming space for movement for the drive magnet; an electromagneticdrive with yoke components encompassing the piston with coils located tothe side of the central yoke; and a can peripherally delimiting therecess.
 2. The drive according to claim 1, wherein the can isconstructed of a material with good sliding properties.
 3. The driveaccording to claim 1, wherein the can is constructed of a ferromagneticmaterial.
 4. The drive according to claim 3, further including: statorpermanent magnets which delimit the recess in an axial direction andoutside the central yoke zone, the can (34) extending into the area ofthe stator permanent magnets.
 5. The drive according to claim 4, whereinthe stator permanent magnets (13, 14) are magnetized such that theyexert a repelling force on the drive magnet.
 6. The drive according toclaim 3, wherein the wall thickness of the can at least outside thecentral yoke zone is so selected, that the drive magnet magnetizes therespective can section to the saturation point when said magnet islocated in the zones outside the central yoke.
 7. The drive according toclaim 6, wherein the can has in the zone of the central yoke a greaterwall thickness.
 8. The drive according to claim 1, wherein the drivemagnet is a rare earth magnet.
 9. An operating method for an oscillatingpiston drive with the characteristics of claim 1, wherein current flowthrough the coils is controlled such that current passes through onlyone coil at a time.
 10. An operating method for an oscillating pistondrive for a vacuum pump including a piston with two piston sections anda central zone equipped with a drive magnet, cylinder sectionsassociated to the piston sections, an annular recess defined between thecylinder sections adjacent the central yoke, said recess forming spacefor movement for the drive magnet, and an electromagnetic drive withyoke components encompassing the piston with coils located to the sideof the central yoke, the method comprising: supplying an alternatingcurrent to the coils with only one current polarity being supplied toeach of the coils.
 11. The operating method according to claim 10,wherein the current flows through the coils such that they alternatelyexert repelling forces on the drive magnet.
 12. The operating methodaccording to claim 10 further including: adjusting amplitudes of thecurrent flows to each of the coils with a thyristor regulator.
 13. Theoperating method according to claim 12, further including: adjustingcurrent amplitude to the coils by means of a phase angle regulator. 14.The operating method according to claim 13, wherein an input signal forthe phase angle regulator is one of: defined at a fixed value for agiven pump application, dependent on the mains voltage, defined by asensor for detecting a position of the piston, or defined by a sensorfor observing valve movement.
 15. The operating method according toclaim 13, wherein the current amplitude is adjusted with thyristorregulators.
 16. The oscillating piston drive according to claim 1,further including: thyristor regulators for controlling current flow tothe coils.
 17. The oscillating piston drive according to claim 1,further including: a means for supplying an alternating current to thecoils with only one current polarity being supplied to each of thecoils.
 18. The oscillating piston drive according to claim 17, whereinthe means for supplying alternating current to the coil includes: aphase angle regulator and a means for supplying an input signal to thephase angle regulator which input signal is one of: a fixed value for agiven pump application, dependent on a voltage of supply means, definedby a sensor for detecting a position of the piston, or defined by asensor for observing valve movement.